Atomic force microscopy (AFM) is a technique for measuring surface topography. AFM typically relies on repulsive forces between a specimen substrate and an AFM probe tip to detect extremely small surface variations. To measure topography of a particular surface, the probe tip, which is fabricated at the end of a cantilever, is brought into contact with the surface and is scanned over a predetermined area of the surface. The deflection of the cantilever is measured (e.g. by a laser beam and photodetector or interferometer) and is correlated to heights of surface features. This information is electronically recorded as a topographical image of the scanned area.
There are two different prevalent modes of operation for AFM, namely contact mode and tapping mode (also referred to as TappingMode.TM. which is a trademark of Digital Instruments). In the contact mode, a probe tip is brought into contact with the surface to be scanned and remains pressed against the surface throughout the entire scanning procedure. However, when measuring surfaces of certain materials, particularly insulating materials, the contact mode often fails to provide an accurate representation of the surface and is not repeatable. In the contact mode, the probe tip is more or less rubbed across the surface, creating frictional and shearing forces that damage the AFM probe tip as well as the sample under investigation. Therefore, a generated image of a surface topography may not accurately depict the actual specimen surface.
One attempt to solve the problem of the interaction between the probe tip and the specimen surface is to use the tapping mode. In the tapping mode, rather than keeping the probe tip constantly in contact with the specimen surface, the probe tip is repeatedly and continuously tapped against the surface during the scanning process. The cantilever and integral tip are oscillated at the cantilever's fundamental frequency. The tip is brought into contact with the surface, thereby damping the amplitude of the cantilever's oscillation. The amount of damping is held constant with a feedback loop by moving the tip and lever or the sample up or down as the tip traces the surface topography. The distance which the lever and tip or sample is moved in order to keep the damping constant is recorded and is directly related to heights of surface features. In this way, topography of the sample is measured. Because the tapping mode eliminates the constant rubbing, which produces frictional and shearing forces between the probe tip and the specimen characteristic of the contact mode, it was initially believed that the resulting images would be better representative of the actual specimen surface.
However, even in the tapping mode, reproducibility and accuracy of the AFM scan is often lacking. Therefore, a need exists for an improved method for using AFM to measure surface topography. Such a method would be beneficial in numerous AFM applications. One application in particular would be measuring the quality and integrity of thin films, such as gate oxides, in semiconductor devices.